Dielectric heating apparatus



Aug- 16, 19 E. w. A. SMITH ET AL 2,949,523

DIELECTRIC HEATING APPARATUS Filed Oct. 17, 1957 4 Sheets-Sheet 1 ZQ/W? ($192145; @Wtomey Aug. 16, 1960 E. w. A. SMITH ETAL 2,949,523

DIELECTRIC HEATING APPARATUS 4 Sheets-Sheet 2 Filed Oct. 17. 1957 ttorney 1960 E. w. A. SMITH ET AL 2,949,5523

DIELECTRIC HEATING APPARATUS Filed Oct. 17, 1957 4 Shets-Shem-in a;

LOAD C IRCU/ '7' FREQUENCY ttorney Inventors ttorn e y 4 Sheets-Sheet 4 C Wwi Aug. 16, 1960 E. w. A. SMITH ET AL DIELECTRIC HEATING APPARATUS Filed Oct. 17, 1957 United States Patent DIELECTRIC HEATING APPARATUS Eric W. A. Smith, Harpenden, and Frank A. Rowley,

Luton, England, assignors to General Motors Corporation, Detroit, Mich., a corporation of Delaware Filed Oct. 17, 1957, Ser. No. 690,728

6 Claims. (Cl. 219-1057) This invention relates to dielectric heating apparatus.

In such apparatus, it is desirable for the work or load circuit, i.e. that which includes the heating electrodes and the dielectric between them, to be tuned to the resonant frequency of the thermionic generator which supplies the radio-frequency current. Departure from the tuned condition occurs if the capacitance of the load changes. This may arise from change in the dielectric constant of the material during treatment, due to the effect of the heat; or, if the apparatus is a press, reduction of the distance between the electrodes under the applied pressure, as the material softens under the heat. The load circuit may also be initially out of tune, due to the treatment of material of different thicknesses, or which have different dielectric constants before treatment. 7

By the present invention, detuning of the load circuit is automatically corrected, from whichever of the above causes it may arise.

The inventors have discovered that the relation between the generator anode current (which is applied to the work), and one phase of the current in the input line to the generator, is substantially linear, and in the particular case investigated corresponds to the law:

where IP is the input line current and Ia is the anode current.

Thus the generator input current, which is of low frequency and can easily be measured, is an indication of the anode current and the related load current, which latter being of radio-frequency can less conveniently be measured.

According to the present invention there is inserted in the input to the generator circuit a current-sensitive meter adapted, by response to current change, to actuate the automatic adjustment of a device which varies an electrical characteristic of the load circuit so as to bring that circuit substantially into tune, or maintain it substantially in tuned condition.

The adjustable device associated with the load circuit may be in the form of a variable inductor, which varies the inductance of the load circuit; or it may be in the form of a variable condenser which applies a capacitance so proportioned-as to counteract changes in the capacitance of the dielectric so that the total capacitance of the load circuit remains substantially unchanged. Whatever the nature of the adjusting device, it may be driven by a reversible electric motor energised by a relay responsive to the reading of the current-sensitive meter.

By way of example the invention will be further described in terms of dielectric heating presses applicable to the manufacture of laminated material incorporating thermoplastic synthetic resin sheet which undergoes compression during the heating treatment. Such presses may be used in the manufacture of laminated trim material for decorative panelling or for upholstering furniture.

In making such laminated materials, the thermoplastic substance itself may cause the layers to adhere together.

The pressure between the platens may impress a decorative pattern into the material, and matters may be so arranged that the adherence of layer to layer o'ccurs only along the impressed lines or areas of that pattern.

The initial tuning of the load circuit is preferably to a frequency which is below that of the generator, so that in the case of a variable inductor, .the driving electric motor is immediately started when the radio-frequency current is switched on, and runs in the direction which reduces the inductance until the frequency of the load circuit is brought up substantially to that of the generator, when the motor stops. The motor runs fast enough for the effect of the decreasing inductance to overtake rapidly the opposing effect of any increasing capacitance caused by compression of the thermoplastic material, so that the load circuit is brought into tune in the space of a few seconds. Should further compression of the work material occur, due to further softening of the thermo plastic material, the motor will operate again in response to the drop in the anode, and hence in the supply, current. In the case of a variable motor-driven condenser, the operation would be similar: in both cases theeffect of current (and hence frequency) fall below a given limit operates an under-current relay which energises the motor to run in the direction which has the effect of raising the frequency of the load circuit.

The circuit arrangement may be such that, when the radio-frequency current is switched off, the motor is energised to run in the reverse direction and returns the inductor or other variable device to its initial position which gives the required tuning of the load circuit to a frequency sufficiently below the generator frequency.

Where the variable device is an inductor, this may comprise a pair of metal bars along which is movable a slider which varies the length of the bars included in the load circuit. The slider may be moved by a threaded rod rotatable by the motor.

The scope of the invention is defined by the appended claims; and how it may be performed is hereinafter particularly described with reference to the accompanying drawings in which:

Figure l is a perspective side view of a dielectric press apparatus according to the invention;

Figure 2 is a diagrammatic circuit diagram of the electrical connections of the apparatus;

Figure 3 is a graph showing the relation between the anode current and load circuit frequency;

Figures 4 and 5 are respectively a side elevation and plan of part of the apparatus; and

Figure 6 shows a detail of the part of the apparatus shown in Figures 4 and 5.

The press apparatus shown in the drawings is intended for making laminated trim panels comprising two or three layers of material, one of which (that which is intended to be the outer surface in use) is composed of a thermoplastic synthetic resin material such as polyvinyl chloride. The second layer (forming the back in use) may bea stiffening board, and there may be a layer of padding between the two outer layers. Such trim material may be used in the interior bodywork of motor vehicles.

The press 10 (Figures 1 and 2) comprises two platens 12 and 14 of which the upper one 12 is movable by an hydraulic ram (not shown), so that the assembly 16 (Figure 2) of sheets is pressed between the platens while it is being subjected to dielectric heating by the application to the platens of a radio-frequency potential.

The radio frequency potential is generated by a thermionic valve generator 2t) (Figure 2) which may be of any convenient type which has, in its anode current wave-form a'component of the desired radio-frequency for applying the necessary potential to the platens 12 and 14 of the press.

The generator is housed in a control box 22 and issupplied from a 440 volt AC. 3 phase supply 24 through a main isolator switch 26 and a step-up transformer 28. The control box 22 also contains a stepdown transformer 30, a control switch 32, and a relay 34 for energising the step-up transformer 28 when the switch 32 is made.

When the isolator switch 26 is made power is supplied to the primary winding 36 of the transformer 30, the winding 36 being in parallel with the primary winding 38 of the transformer 28, which is not, however, energised until contacts 39 of the relay 34 are closed. The secondary winding 40 of the step-down transformer is such that the 440 volt 3 phase supply is converted into a single phase 250 volt supply, and is in a circuit with the switch 32 and energising coil 42 of the relay 34. The switch 32 is controlled by a timing device (not shown) which also controls the hydraulic ram for the platens 12 and 14. Thus when the timing device is operated the switch 32 is closed so that the 440 volt supply is connected to the primary winding 38 of the transformer 28 whose secondary Winding 43 is such that the voltage is stepped-up to 6000 volts and which is connected through a rectifier system 44 (shown only very diagrammatically in Figure 2) to the generator 20.

The generator 20 supplies a load circuit which includes a variable inductor 46 in parallel with the press platens 12 and 14. The position of the feed point, that is the connection of the generator 20 to the press platens 12 and 14, is adjusted so that the load impedance is equal to the source impedance, thus ensuring maximum power transfer when the load is turned to the generator.

When the timing device is operated the switch 32 is closed and the hydraulic ram causes the platens 12 and 14 to apply pressure to the assembly 16 which is therefore heated. The heating is due to the effect of the loss of the in-phase component of the current through the assembly 16, which forms the dielectric of a capacitor, the plates of which are the platens 12 and 14 of the press. The platens have a pattern of parallel ridges 39, which when the platens are brought together impresses a similar pattern into the polyvinyl chloride sheet and the layer or layers underneath it. The thermoplastic top sheet, and also any other thermoplastic substance which may be present between the sheets, softens along the lines of the impressed pattern and the sheets are thus bonded together along these lines. Due to the softening of the thermo-plastic material during the heating process, which may last from a few seconds upwards, the platens 12 and 14 approach each other more closely, thus increasing the capacitance of the dielectric during the heating process. At the same time the dielectric constant changes due to the effect of heat. Variations in capacitance may also be caused initially if the assembly of sheets is of different thickness from normal, or if the dielectric constant of the material is different.

By alterations in capacitance due to any of these causes, the load circuit comprising the platens 12 and 14, the assembly 16 and the inductor 46 may become out of tune in relation to the circuit of the thermionic generator 20, thereby causing heating time to be unnecessarily long.

Automatic tuning or returning of the load circuit is brought about by adjustment of the variable inductor 46 in response to current drop in the 446 volt supply 24. As mentioned above, it has been found that there is a substantially linear relation between the load or anode current of the generator 20 and the input current of the ordinary supply 24 to the generator. Therefore, a drop in the anode current from the generator 20 is accompanied by a related drop in the current of the supply 24 into the generator. A well damped moving coil ammeter is used for sensing this current drop, and, to

4 avoid passing large currents through the coil, a transformer 48 is interposed between one phase of the supply 24 and the coil of the ammeter. The ammeter has high and low adjustable contacts 50 and 52 between which the ammeter needle 54 moves. (In Figure 2 only the needle 54 and contacts 50 and 52 are shown). Movement of the ammeter needle 54 controls the operation of relays 56 and 57 which in turn control a motor 58 operative to adjust the variable inductor 46.

The relays 56 and 57 are operated by a 20 volt D.C.

supply provided through a step-down transformer 60 and rectifier 62 from the secondary winding 40 of the transformer 30. The operating circuits of the relays 56 and 57 include energising coils 64 and 66 respectively, the former being energised when the needle 54 contacts the low ammeter contact 52 and the latter when the needle contacts the high ammeter contact 50. When the coil 64 is energised a pair of contacts 68 are closed, and when the coil 66 is energized a pair of contacts 70 are closed, the contacts 68 and 70 being biassed so that they are open when the corresponding coil is not energised.

The motor 58 is a reversible induction motor of splitphase, capacitor-start-and-run, type, and is supplied from the secondary winding 40 of the transformer 30 through either the contacts 68 or 70' of the relays 56 and 57, and contacts 72 or 74 of two similar micro-switches 76 and 78 respectively, whose function is described later. When the ammeter needle 54 contacts the low contact 52 and the coil 64 is energised the motor turns to reduce the inductance of the inductor 46, and when the needle contacts the high contact 54 the motor turns to increase the inductance of the inductor.

The inductor 46, which will be described in more detail later with reference to Figures 4 to 6, consists essentially of a pair of copper bars 80 and 82 arranged parallel to each other and having movable along them a slider 84 which determines the length of the bars which is included in the load circuit. The longer the length of the bars 80 and 82 so included, the greater the amount of the inductance. Movement of the slider 84 along the bars 80 and 82 is produced by means of a threaded rod 86 arranged parallel to the bars and which is inserted through a tapped hole in the slider; the rod 86 is rotatable by an insulated drive 88 which forms a co-axial continuation of the shaft of the motor 58.

The essentials of the functioning of the automatic control will now be described.

The initial tuning of the load circuit, comprising the platens 12 and 14 and inductor 46, is such that its resonant frequency is below that of the circuit of the generator 20. When the radio-frequency current is switched on after the press 10 has been closed with the sheet material in it, the current initially detected by the ammeter closes the contacts 68 if the frequency of the load circuit is too far below what is required, and the motor 58 rotates in the direction which reduces the inductance of the variable inductor 46 (that is moves the slider 84 to the right in Figure 2), so that the load circuit is rapidly brought into tune. When the thickness of the sheet material 16 is reduced during heating, due to compression and softening of the thermoplastic material, there is an increase in capacitance; however, the motor 58 is arranged to run fast enough for the corrective action of reducing inductance to overtake the opposite eifect of increasing capacitance. When the current value detected by the ammeter reaches a value slightly below that obtained when the load is tuned to the generator 20 the motor 58 stops, since the needle 54 of the ammeter moves clear of the low contact 52 to de-energise the coil 64 so that the con tacts 68 open. If there is any further compression of the thermoplastic material during treatment then the motor 58 will restart to correct it due to the load current dropping so that the needle 54 engages thev low contact 52, There may in fact be several starts and stops of the motor 58 during one period of heating.

The load circuit frequency must not be increased, by adjustment of the inductor 46, above the frequency of the generator circuit, that is there must be no over-correction. If over-correction should occur, the generator anode current (and therefore that detected by the ammeter) would begin to fall once again; if this fall were of such a magnitude that the needle 54 contacted the low contact 52 the motor 58 would effect a further reduction in the inductance, which would carry the load circuit still further out of tune.

This is shown in Figure 3 where the ordinate represents anode current, the abscissa, the load circuit frequency, and the line parallel to the ordinate-the generator frequency. The curve shows clearly that the anode current increases with higher load circuit frequency until the load and generator circuits are tuned to each other. Thereafter any increase in load circuit frequency results in a reduction in anode current.

Over-correction is prevented by the ammeter needle 54 contacting the high contact 50. This energises the coil 66 of the relay 57 so as to close the contacts 70, and thus cause the motor 58 to rotate in a direction such that the slider 84 moves to increase the inductance of the inductor 46, that is to the left in Figure 2. However, in actual operation the high contact 50 and relay 57 have been found unnecessary, since, when the generator circuit is tuned, the motor 58 stops within a very few revolutions. To allow for the motor 58 coming to a standstill, the low contact 52 is adjusted so that the ammeter needle 54 moves clear of it to open the motor circuit just before exact tuning is reached.

In order to ensure that the inductance of the inductor 46 is high in relation to the capacitance of the press so that the frequency of the load circuit is below that of the generator circuit at the beginning of a heating period, there is a relay 90 operated when the radio-frequency current is switched off. The relay 90 includes an energising coil 96 supplied with 250 volts A.C. by the secondary winding 40 of the transformer 30, and -a pair of contacts 94 in parallel with the contacts 70. Whenever the switch 32 is closed the coil 96 is energised and then serves to hold the contacts 94 open. When the switch 32 is opened, however, to switch off the radio-frequency current, the contacts 94 close so that the motor 58 turns to move the slider 84 to the left (Figure 2) to increase the inductance.

After the inductor 46 has been reset, the motor 58 is stopped by the micro-switch 76, which is actuated by a limit rod 98. The limit rod 98 is parallel to the threaded rod 86 and carries two adjustable stop members 100 and 102, and a plate of insulator material 104 which is disposed between spring-biassed operating plungers of the micro-switches 76 and 78. When the motor 58 has moved the slider 84 sufiiciently to reset the inductor, the slider engages the stop 100, moves the rod 98 to the left and so opens the contacts of micro-switch 76, which stops the motor.

Since the inductor 46 has been reset, the motor 58 will, when the switch 32 is made at the beginning of the next heating period, move the slider 84 to the tight (Figure 2) so that it moves away from the stop 100. The bias of the spring of the operating plunger of the micro-switch 76 then closes the contacts 72 and resets the limit rod 98 by moving it to thevright.

The slider 84, if it should engage the stop 102, which it would do if an unusually thin assembly 16 were being pressed, moves the limit rod 98 to the right so as to actuate the other micro-switch 78 which operates similarly to the micro-switch 76. Thus once one of the micro-switches 76 or 78 has operated and the motor 58 stopped, the motor can only be restarted in the opposite direction to that in which it was rotating prior to stoppage.

The micro-switches 76 and 78 thus act as safety devices to prevent excessive movement of the slider 84. A further safety device, in case either of the microswitches fails, is that the thread on the threaded rod 86 runs out" short of each end of the rod; the arrangement is such if the slider 84 reaches an extreme position without the appropriate stop 100 or 102- being engaged by the slider, the axial movement of the latter will cease when it reaches the end of the thread and runs on to the plain part of the rod.

The variable inductor 46, which is shown in perspective in Figure 1 and diagrammatically in Figure 2, is shown in greater detail in Figures 4 to 6. The same reference numerals are used for corresponding parts in all the figures.

The inductor 46 and motor 58 are carried by a plate 104 which is secured to the undersurface of the movable half of the press which carries the upper platen 12. The copper bars and 82 have out-turned ends 106 which are fixed to vertical walls 108 and 110 rigid with the plate 104. The limit shaft 98 and threaded rod 86 for the slider 84 are both journalled in the walls 108 and 110, the latter being connected to the motor 58 by the insulated drive 88. Both the drive 88 and the limit shaft 98 are also journalled in a third vertical wall 112.

The slider 84 (Figure 6) consists essentially of a rectangular dunalumin block having a threaded opening 114 through which the threaded rod 86 passes, and two rectangular recesses 116 and 118 in its upper surface. The outer vertical walls of the recesses engage the outer vertical sides of the copper bars 80 and 82. There is, disposed in each of the recesses, a contact member 120 which is forced outwardly against the inner vertical sides of one of the bars 80 or 82 by a spring 122. The spring is a Phosphor bronze strip having S-shaped ends 124 which engage the inner vertical walls of the recesses and the inner sides of the contact members 120. The spring thus ensures good electrical contact between the slider and the bars. Beneath the threaded opening 114 there is a spring loaded plunger 124 which, when the motor 58 has rotated sufficiently, engages one of the adjustable stop members or 102 as described above.

The micro-switches '76 and 78, which are located to one side of the limit shaft 98, are secured to the vertical walls and 112 respectively. The limit shaft 98 carries, between these two walls, the plate of insulator material 104, which when the shaft moves, engages a spring-biassed plunger 126 of one of the micro-switches as described above.

To permit of manually-controlling the motor 58, for testing or setting purposes, two control buttons are provided, one of which controls running, in one direction, of the motor and the other in the other direction. Protection against misuse is provided by connecting the normally-closed contacts of-each bush button switch in series with the normally open contacts of the other, thus preventing the motor from passing any current if both buttons are pressed at the same time. The manual control circuit has been omitted from Figure 2 for the sake of simplicity.

The variable inductor, or other equivalent corrective device, may, instead of being in parallel with the platens 12 and 14, be arranged in series with them.

We claim:

1. Dielectric heating apparatus comprising a generator of radio-frequency current, a power supply input circuit connected with the generator, a load circuit, dielectric heating electrodes forming part of the load circuit, a tuning device in the load circuit, an electric motor to drive said device, an ammeter in said input circuit, a relay in the motor circuit adapted to be operated by the ammeter to start said motor at a given ammeter reading.

2. Dielectric heating press, comprising a generator of radio-frequency current, a power supply input circuit connected with the generator, a load circuit, dielectric heating and pressing platens forming part of the load circuit, a tuning device in the load circuit, an electric motor to drive said device, an ammeter in the said input circuit, a relay in the motor circuit operable to start the motor running in the direction to tune the load circuit, and contacts in the ammeter adapted to operate said relay in the motor runni'ng sense below a pre-determined value of input current.

3. Dielectric heating press, comprising a generator of radio-frequency current, a power supply input circuit connected with the generator, a load circuit, dielectric heating and pressing platens forming part of the load circuit, a variable inductor in the load circuit, an electric motor to drive said inductor, an ammeter in said input circuit, a relay in the motor circuit operable to start the motor running in the direction to reduce the inductance of the load circuit, and contacts associated with the ammeter and adapted to operate said relay in the motorrunning sense below a predetermined value of input current approximating to a tuned condition of the load circuit with the generator.

4. A press according to claim 2, in which the inductor comprises a pair of parallel contact bars, a slider movable along said bars and having a tapped hole, and a threaded rod screwed into said hole and rotatable by the electric motor.

5. A press according to claim 4, having a pair of .lirnit switches adapted to be operated, to stop the motor, by extreme movement of said slider in either direction.

6. Dielectric heating apparatus comprisinga generator of radio-frequency current, a power supply input circuit connected with the generator, a load circuit, dielectric heating electrodes forming part of said load circuit, a variable inductor in the load circuit, and an amm'eter in said input circuit adapted to actuate said inductor so as to tune the load circuit to the resonant frequency of the generator, said inductor comprising a pair of parallel contact bars, a slider movable along said bars and having a tapped hole, and a threaded rod screwed into said hole and rotatable by the electric motor.

References Cited in the file of this patent UNITED STATES PATENTS Reifel et al Feb. 11, 1947 Moore Oct. 11, 1949 Livingston Dec. 20, 1949 FOREIGN PATENTS Great Britain J an. 3, 1949 Great Britain Mar. 23, 1949 

